While dormant trees may not be so easy on the eyes for us, overwintering insects still find their look appealing. Late in the summer, scale insects, mites and aphids will lay eggs on trees that stay on through the winter months until new larvae are born. These larvae can damage fruit trees, such as apple and pear trees, as well as certain shrubs and woody ornamentals.
Thankfully, you can help protect your trees and shrubs with dormant oils (also known as horticultural oils), a readily available and relatively inexpensive solution. An application of dormant oil will help control overwintering insect populations by coating the insects’ spiracles, effectively smothering future larvae. Best of all, the oils are less toxic to beneficial insects, such as ladybugs, as well as birds and mammals.
Applying dormant oil will help to keep your trees and shrubs healthy and give you a head start on insect management in the spring.
Timing is everything. Dormant oils should not be applied until a tree has gone completely dormant, but also before new growth occurs. Applications in late winter or early spring are ideal, when temperatures are above freezing (over 40 degrees is ideal), but also below 70 degrees. Applying the oil too early may expose your tree to winter damage. Applying too late will damage new growth.
Coordinate with other applications. If you also apply a sulfur-containing pesticide, be sure to space out your applications. A combination of dormant oil and a sulfur-containing pesticide can create a compound poisonous to plants. To play it safe, wait 30 days after a sulfur pesticide application before using dormant oil.
Check the label. When choosing a dormant oil, look for a low-viscosity oil, which will spread more easily. Make sure the product you choose is specifically marked as a dormant oil, as other oil types can damage trees and shrubs. Also, be aware of the types of trees and shrubs your dormant oil is formulated to protect. A dormant oil for your apple tree might be harmful for a woody ornamental elsewhere on your property.
Apply with caution. Follow all instructions on the label when applying a dormant oil and be aware of your tree or shrub’s surroundings. If you aren’t sure how or when to apply, contact a local, licensed professional. Nearby annual flowers, in particular, can be damaged.